In this type of application, the goal is to manufacture steel parts that combine high mechanical strength, high impact strength, good corrosion resistance and good dimensional accuracy. This combination is particularly desirable in the automobile industry, where attempts are being made to significantly reduce the weight of the vehicles. Anti-intrusion and structural parts, as well as other parts that contribute to the safety of automotive vehicles such as bumpers, door or center pillar reinforcements, for example, require the above mentioned characteristics, for example. This weight reduction can be achieved in particular thanks to the use of steel parts with a martensitic or bainitic-martensitic microstructure.
The fabrication of parts of this type is described in prior art publications FR2780984 and FR2807447, according to which a blank cut in a steel sheet for heat treatment and pre-coated with a metal or metal alloy is heated in a furnace and then hot formed. The pre-coating can be aluminum or an aluminum-based alloy, zinc or a zinc alloy. During the heating in the furnace, the pre-coating provides protection for the surface of the steel against decarburization and the formation of calamine. During the heating in the furnace, this pre-coating becomes alloyed with the steel substrate to form a compound suitable for hot forming and that does not cause any deterioration of the tooling. Holding the part in the tooling after forming has been performed makes possible a rapid cooling that leads to the formation of hardened microstructures that have very high mechanical characteristics. A process of this type is known as press hardening.
As a rule, the mechanical characteristics of the parts obtained in this manner are evaluated by means of tensile strength and hardness tests. The above referenced documents also describe fabrication processes that make it possible to obtain a mechanical strength (or maximum tensile strength) Rm of 1500 MPa starting with a steel blank that has an initial strength Rm of 500 MPa before heating and rapid cooling.
However, the service conditions of certain hardened and coated parts require not only a high level of strength Rm but also good bendability. This parameter does in fact appear to be more pertinent than the measured elongation at failure in traction to guarantee that the part has sufficient ductility to absorb deformations or impacts without risk of rupture, in particular in the areas corresponding to local stress concentrations due to the geometry of the part or to the potential presence of micro-defects on the surface of the parts.
Document WO2009080292 discloses a process that makes it possible to increase the bending angle of a hardened part. According to this process, a steel sheet is heated in an annealing furnace to a temperature between 650 and 800° C. to obtain a layer of oxide that is significantly thicker than 0.3 micrometers. Certain alloy elements of the steel are oxidized underneath this oxide layer. This oxide layer is then partly reduced so that it has a thickness greater than 0.3 micrometers. The extreme surface of the reduced oxide layer consists of pure iron. The sheet is then coated using a hot-dip process. After this step, the sheet has the following different layers in succession: the steel substrate comprising the oxidized elements in the vicinity of the surface (internal oxidation), this substrate being topped by a partly reduced oxide layer, which is itself topped by the coating applied using a hot-dip process. During the subsequent step of the austenitization of the blank and/or during the shaping and cooling, a thin ductile layer is formed under the coating such that the cracks formed during the coating propagate less easily into this underlying layer during the forming process.
However, the layer of oxides that is present when the sheet is immersed in the metal coating bath can have an undesirable effect in terms of the adherence of the hot-dip coating to this layer.
It would therefore be desirable to have a fabrication process that does not have this disadvantage and that would make it possible to obtain simultaneously, after press hardening, high levels of tensile strength and bendability.
It is also known that industrial fabrication conditions inevitably include a certain variability such as, for example, of the temperature cycle during the annealing of the sheet before it is coated, and the composition and/or the dew point of the atmosphere of the continuous annealing furnaces, which can vary slightly during a given fabrication sequence or can vary from one fabrication run to another. Even if the maximum precautions are taken to minimize these variations, it would be desirable to have a fabrication process such that the mechanical characteristics, and in particular the bendability, obtained after press hardening are as insensitive as possible to this potential variation of the fabrication conditions. An additional objective is a fabrication process that results in good isotropy of the parts after hot stamping, i.e., in which the bendability is not highly dependent on the direction of stress in relation to the direction in which the sheet is rolled.
It is also known that the hold time of the blanks in the furnace during the austenitization step during hot stamping can influence the mechanical characteristics of the parts. It would therefore be desirable to have a fabrication process that is less sensitive to the hold time in the furnace to achieve a high level of reproducibility of the mechanical characteristics of the parts.
In the case of parts fabricated from sheets pre-coated with zinc or zinc alloy, the objective is to have a process that makes it possible to weld these parts without the risk of embrittlement of the grain boundaries caused by penetration of liquid zinc.